The invention relates to a color display tube comprising in an evacuated envelope an electron gun system for generating three electron beams which are situated with their axes in one plane and which in the operating tube are deflected over a display screen in two mutually perpendicular directions, which plane extends in a first direction of deflection, said display screen comprising patterns of areas luminescing in different colors, there being provided in front of the said display screen a color selection electrode which comprises rows of substantially rectangular apertures, the rows extending substantially parallel to the second direction of deflection and the apertures in each row being separated from each other by bridges, said apertures having a substantially equal pitch in the direction of the rows, and the apertures of juxtaposed rows being staggered with respect to each other.
Such a colour display tube is known from U.S. Pat. No. 3,973,159. This Specification discloses a color display tube having a colour selection electrode, sometimes termed shadow mask, comprising parallel rows of apertures. These rows of apertures are staggered in known manner in n steps with respect to each other so as to reduce moire-patterns. Moire-patterns are bright and dark bands which are formed in the displayed picture by interference of inter alia the lines of the field from which the picture is built up with the pattern of the apertures in the colour selection electrode. Both the lines of the field and the pattern of the apertures show a periodicity from which the occurrence of the bright and dark bands by interference can simply be explained. In order to further reduce the moire-patterns, the above-mentioned U.S. Pat. No. 3,973,159 suggests choosing the space between the apertures in the direction of the rows to be equal to p/n, wherein p is the pitch in the direction of the rows between the apertures. However, this measure has not proved to be totally satisfactory in practice.
It has moreover been found that for a color display tube which is destined for use in one special transmission system and in which a given number of scanning lines per picture and consequently a given line spacing occurs, the mutual space between the apertures in the colour selection electrode should be chosen in a specific manner in relation to the line spacing so as to minimize moire-effects, if any. However, in these optimum conditions, too, moire-effects which are still visible may occur with a small dimension of the spot of the electron beam on the display screen in the second direction of deflection. In colour display tubes of the "in-line" type, this is the case especially on the left-hand and on the right-hand sides of the display screen. The possibility that moire-effects are still observed exists also if deviations occur as a result of the non-linearity of the picture time base. Furthermore, the choice of the space between the apertures is restricted because different line spacings have to be taken into account if the display tube is to be suitable for different transmission systems.
In order to mitigate these disadvantages, U.S. Pat. No. 3,486,061 suggests making the smallest angle between the second direction of deflection and the rows of apertures in the color selection electrode between 5.degree. and 10.degree.. The occurrence of visible moire-patterns is then strongly reduced. In the color display tube described in the said Patent Specification, three electron beams are generated which are situated with respect to each other such that in the electron gun system the cross-sections of the axes of the beams with a plane perpendicular to the tube axis constitute the corners of an equilateral triangle. This is a so-called "delta" gun arrangement. One side of the said triangle is substantially parallel to the scanning lines. It is of importance that the relationship between the location of the electron beams and the rows of, in this case circular, apertures in the colour selection electrode is maintained. Since according to the U.S. Pat. No. 3,486,061 the smallest angle between the direction at right angles to the scanning lines and the rows of apertures in the colour selection electrode is between 5.degree. and 10.degree., this should hence also apply to the smallest angle between the direction of the scanning lines on the display screen and the side at the triangle formed by the electron beams in the electron gun system. This can be achieved by rotating, during the assembly of the color display tube, the electron gun system about its axis over the relevant angle between 5.degree. and 10.degree. with respect to the usual assembly position.
The rotation of the electron gun system in a colour display tube of the "in-line" type, in which three electron beams are generated which are situated with their axes in one, usually horizontal, plane, is not attractive since this will present problems in convergence.
It is therefore an object of the invention to provide an "in-line" colour display tube of the kind described in the opening paragraph in which the occurrence of moire-patterns is effectively suppressed and in which it is not necessary to rotate the electron gun system.